In this question, habríamos comprado should aalso be a correct answer.

I think my typing was right. I remembered checking the accent on the letter í. I am pretty sure the system told me it was wrong.

There are 2 right answers listed for this question, but they are in drop-down format instead of ticj boxes so only one may be selected at a time. 

Si me hubieseis dicho que veníais, nosotros ________ unas cervezas.

If you had told me you were coming we would have bought some beers.

It technically marked me correct but it docks me about 15 percentage points instead of adding points when I use "habríamos comprado" instead of "hubiésemos comprado"